The transcription factor hepatocyte nuclear factor 1 (HNF1) is a tissue-specific regulator that also plays an essential role in early development of vertebrates. In humans, four heterozygous mutations in the HNF1 gene have been identified that lead to early onset of diabetes and severe primary renal defects. The degree and type of renal defects seem to depend on the specific mutation. We show that the frameshift mutant P328L329fsdelCCTCT associated with nephron agenesis retains its DNA-binding properties and acts as a gain-of-function mutation with increased transactivation potential in transfection experiments. Expression of this mutated factor in the Xenopus embryo leads to defective development and agenesis of the pronephros, the first kidney form of amphibians. Very similar defects are generated by overexpressing in Xenopus the wild-type HNF1, which is consistent with the gain-of-function property of the mutant. In contrast, introduction of the human HNF1 mutant R137-K161del, which is associated with a reduced number of nephrons with hypertrophy of the remaining ones and which has an impaired DNA binding, shows only a minor effect on pronephros development in Xenopus. Thus, the overexpression of both human mutants has a different effect on renal development in Xenopus, reflecting the variation in renal phenotype seen with these mutations. We conclude that mutations in human HNF1 can be functionally characterized in Xenopus. Our findings imply that HNF1 not only is an early marker of kidney development but also is functionally involved in morphogenetic events, and these processes can be investigated in lower vertebrates.
The tissue-specific transcription factor HNF1alpha is expressed in kidney, liver, intestine and stomach of Xenopus. We show that the HNF1alpha gene is transcriptionally activated at the onset of zygotic gene transcription and that this transcription is maintained throughout development. Ectodermal explants of blastulae (animal caps) express HNF1alpha mRNA upon stimulation with the mesoderm inducers activin A and BMP4 as well as on overexpression of Smad2 and Smad1, the corresponding members of the intracellular TGF-beta signal transducers, respectively. Beside these factors that mediate their response through serine/threonine kinase receptors, bFGF, which acts via tyrosine kinase receptors, leads to HNF1alpha expression, too. These embryonic inducers result in a delayed appearance of HNF1alpha mRNA, excluding a direct activation of HNF1alpha. In contrast, the maternally expressed nuclear receptors HNF4alpha and HNF4beta activate the initial HNF1alpha transcription, since overexpression of HNF4 leads to a rapid expression of HNF1alpha mRNA in animal caps. Similarly, in entire neurulae HNF4 overexpression results in increased HNF1alpha transcription. Therefore, we assume that the initial activation is dependent on maternal HNF4alpha and HNF4beta transcription factors whereas HNF1alpha induction by growth factors reflects the property of these factors to induce the differentiation of mesodermal and entodermal cell types expressing HNF1alpha.
Tissue-specific transcription factors play an essential role in establishing cell identity during development. We review our knowledge of the molecular events involved in the activation of the gene encoding the tissue-specific transcription factor HNF1 alpha (LFB1). The available data suggest that the maternal factors OZ-1, HNF4 alpha and HNF4 beta act as initial activators of the HNF1 alpha promoter. We present evidence suggesting that the mesoderm-inducing factor activin A plays a critical role by acting through the HNF4 binding site of the HNF1 alpha promoter. The activity of this embryonic morphogen seems to form a gradient opposing the distribution of the maternal HNF4 proteins that are concentrated at the animal pole of the egg. After zygotic gene transcription the HNF1 alpha-related transcription factor HNF1 beta accumulates faster than HNF1 alpha itself and thus is likely to contribute to the activation of the HNF1 alpha transcription via the HNF1 binding site. The cofactor of the HNF1 proteins (DCoH) is present throughout development and thus cannot limit the activation potential of HNF1 alpha in early development. Our results provide a detailed description of setting up the expression pattern of a tissue-specific transcription factor during embryogenesis.
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